In the conventional plant control system, a plant is controlled by a programmable logic controller. This programmable logic controller is connected to a supervisory control device via a scan transmission device. The programmable logic controller and the supervisory control device transfer data from one to another.
In this plant control system, a SCADA system is used for the supervisory control device. In this SCADA system, a plurality of monitoring screens are prepared. On each of the monitoring screens, parts such as control operating buttons and indicating lamps are arranged. Also, in the SCADA system, an internal tag is used as an internal signal. Further, in the SCADA system, an I/O (input/output) tag is used as a signal for transferring data to and from the programmable logic controller.
In the plant control system configured as described above, in some cases, the plant is controlled from the supervisory control device. In this case, an operator depresses the control operating button or the like displayed on the SCADA system. Thereby, an I/O tag corresponding to the control operating button or the like is selected. Thereafter, an acknowledge button such as an execution button is depressed. Thereby, the selected I/O tag is turned ON. The ON-state I/O tag is transmitted to the programmable logic controller.
If this ON state is controlled as it is on the scan transmission device, in some cases, the selected I/O tag does not reach the programmable logic controller on account of wire breaking of the scan transmission device or other causes. In this case, the programmable logic controller misjudges that the selected I/O tag has been turned OFF. To prevent this misjudgment, usually, the programmable logic controller detects a change of the selected I/O tag, and holds the ON state of the selected I/O tag.
The I/O tag held in the ON state must be turned OFF before the next operation of the control operating button or the like. One method for turning off the I/O tag is to execute, in the SCADA system, a program for issuing a command to turn OFF the selected I/O tag after certain time has elapsed, and to deliver a command signal to the programmable logic controller. The output of a command signal for turning OFF the I/O tag after the certain time has elapsed from when the I/O tag was turned ON is called a single shot output.
In the above-described configuration, to hold the selected I/O tag in the SCADA system, an internal tag is needed for each of the control operating buttons. Also, an internal tag for a counter that counts the certain time with the depression of the acknowledge button being the basis is needed for each of the control operating buttons. Therefore, as the number of control operating buttons increases, the number of internal tags increases greatly. Accordingly, the number of programs written in the SCADA system also increases. This presents a problem that the preparation of single shot output requires a lot of programs and internal tags. As the result of this problem, there arise problems of deteriorated performance of the SCADA system, decreased reliability caused by program mistakes, increased amount of work at the time of addition/deletion of signals, and the like. Further, because the number of tags is limited, a large-scale plant control system has a problem that the internal tags are exhausted.
To solve the above-described problems, various proposals have been made. For example, there has been proposed a method in which the SCADA system receives a response signal with respect to signal reception from the programmable logic controller, whereby the signal output from the SCADA system is turned OFF. Thereby, the programs written in the SCADA system can be reduced (for example, refer to Patent Literature 1).